A high speed oscilloscope is one example of means for measuring the waveform or the like of an electrical signal that changes at high speed. The response time (or time resolution) of the high speed oscilloscope is of the order of one nano-second (10.sup.-9 second). On the other hand, the time resolution of a sampling type oscilloscope is of the order of thirty pico-seconds. It should be noted, however, that the sampling type oscilloscope can measure only repetitive phenomena.
It is well known in the art that an ultra-high speed streak camera can measure a light emitting phenomenon that changes at extremely high speed. The streak camera has a time resolution of the order of pico-seconds or better, and is generally used to measure optical signals. When an element for converting an electrical signal into an optical signal, such as a laser diode, is provided in front of the streak camera, then an electrical signal can be measured with a time resolution in the range of nano-seconds to several pico-seconds which is higher than that in the case of the above-described oscilloscope.
The operating principle of an electrical signal measuring method using a streak camera will be described with reference to FIG. 1.
An optical signal is formed with an intensity that is proportional to or in a relation of 1:1 with an electrical signal to be measured. The optical signal is converted into a streak image by means of a streak tube, for measurement of the electrical signal.
A laser diode 901 is driven by the electrical signal to be measured, and emits a light beam. The light beam is applied to a slit plate 902 in an input optical system. The slit image of the slit plate 902 is formed on a photocathode 910 of a streak tube 900.
At the photocathode 910, the slit image is converted into an electronic image, which is accelerated by an accelerating electrode 911 and focused to enter a deflection field.
In the deflection field, a pair of deflection plates 9l2A and 9l2B are provided. When the electronic image passes through the deflection field, a high speed sweep voltage is applied to the deflection plates by a sweep voltage generating circuit 915 so that the electronic image sweeps downwardly. This sweep should be synchronous with the passage of the electronic image. For this purpose, an electrical signal formed from a part of the incident light beam, or a part of the electrical signal to be measured is utilized as a trigger signal.
In a single sweep streak camera, the high speed sweep voltage is similar in waveform to a saw-tooth wave, and therefore the sweep repetition is limited to a maximum of several kilo-hertz (KHz). In a synchro scan streak camera, the sweep voltage is a sine wave synchronous with an electrical signal to be measured which is high in repetition, and the sweep frequency is 75 to 165 MHz.
When the light emission of the laser diode 901 is synchronous with the sweep waveform, a streak image is formed on a phosphor screen 914 at the same position with high repetition. The streak image is integrated, so that the weak optical phenomenon can be measured with high S/N ratio in a short time. The electronic image, after being electron-multiplied by a factor of about 1000 when passed through a micro-channel plate 913, is applied to the phosphor screen 914, where it is converted into an optical image. For analysis, the streak image thus formed is picked up through a relay lens (not shown) with an SIT camera.
The position of the streak image on the phosphor screen 914 depends on when the electrons formed by the incident light beam are emitted from the photocathode 910. that is, the time axis of the incident light beam is converted into the vertical axis of the phosphor screen 914. Accordingly, the time difference can be detected from the position of the streak image on the vertical axis of the phosphor screen 914, and the optical intensity from the density of the image.
The data, in the slit direction, of the slit image on the photocathode 910 remain, as they are, in the streak image in the direction of the horizontal axis. Therefore, with an image-forming system or spectroscope arranged in front of the streak tube 900, the variations of the optical intensity with respect to positions and wavelengths can be measured.
FIG. 2 shows the variations of the data included in the streak image in the horizontal axis and the streak image.
In the above-described measuring method using the streak camera, the time resolution is proportional to the time period for which the observation is permitted (hereinafter referred to as "an observation time period," when applicable). Therefore, as the time resolution is increased, the observation time period is decreased. In an ordinary streak camera, the relation between the time resolution and the observation time period is as follows: EQU (observation time period)=200.times.(time resolution)
For instance when the time resolution is two pico-seconds, then the observation time period is: EQU 2.times.10.sup.-12 .times.200=4.times.10.sup.-10 =0.4 nano-second
Accordingly, it is impossible for only one measurement (sweep) to achieve the measurement of a relatively long optical phenomenon with high time resolution.
In the case of an ultra-high speed IC or ultrahigh speed logic circuit, it is necessary to measure a relatively long time period with high time resolution to measure the timing of the operations thereof and of various pulses. Furthermore, it is necessary to use means for instantaneously recording and analyzing the waveforms of the pulses.